locking.rb

require 'active_support/core_ext/module/delegation'

module Locking

  def lock(*whitelist, &block)
    Class.new(lockable) { delegate(*whitelist, to: :context) }
      .new(self).instance_eval(&block)
  end

  private

  def lockable
    Class.new do
      attr_accessor :context

      def initialize(context)
        @context = context
      end
    end
  end
end

class Foo
  include Locking
  def bar
    puts 'called bar'
  end
end

Foo.new.lock { puts "I can call Kernel and Object methods still." }
Foo.new.lock(:bar) { bar }
Foo.new.lock { bar }

output

$ruby locking.rb
I can call Kernel and Object methods still.
called bar
lock.rb:18:in `block in <main>': undefined local variable or method `bar' for #<#<Class:0x007fd3b394adc0>:0x007fd3b394aa00> (NameError)
	from lock.rb:13:in `instance_eval'
	from lock.rb:13:in `lock'
	from lock.rb:18:in `<main>'